1. Field of the Invention
The present invention concerns a method to suppress unwanted signal components during an acquisition (triggered via a physiological signal of the examination subject) of magnetic resonance measurement data from an examination subject and a corresponding magnetic resonance apparatus, and a non-transitory computer-readable data storage medium to implement such a method.
2. Description of the Prior Art
The MR technique (MR: magnetic resonance) is a technique known for a few decades with which images of the inside of an examination subject can be generated. As a significantly simplified description, the examination subject is positioned in a comparably strong, static, homogeneous basic magnetic field (field strengths from 0.2 Tesla to 7 Tesla or more) in a magnetic resonance apparatus so that nuclear spins in the subject orient along the basic magnetic field. Radio-frequency excitation pulses are radiated into the examination subject to trigger nuclear magnetic resonance signals, the triggered nuclear magnetic resonance signals are measured (detected), and MR images are reconstructed based thereon. For spatial encoding of the measurement data, rapidly switched magnetic gradient fields are superimposed on the basic magnetic field. The acquired measurement data are digitized and stored as complex numerical values in a k-space matrix. An associated MR image can be reconstructed from the k-space matrix, populated with such values, by means of a multidimensional Fourier transformation. The chronological order of the excitation pulses and the gradient fields to excite the image volume to be measured, for signal generation and for spatial encoding is designated as a sequence (or also as a pulse sequence or measurement sequence).
In magnetic resonance imaging (MRI) of examination regions affected by breathing movement, for example the organs of the thorax and abdomen, the breathing movement can lead to ghosting, blurring, intensity loss and registration errors between images in the reconstructed MR images. These artifacts hinder the identification of findings by a physician on the basis of these MR images and can lead to pathological changes (such as lesions, for example) being overlooked.
Numerous techniques exist in order to reduce artifacts as a result of breathing movement. Some of these techniques can be summarized under what is known as respiratory triggering. For example, such techniques are described in the overview article by Craig E. Lewis et al. “Comparison of Respiratory Triggering and Gating Techniques for the Removal of Respiratory Artifacts in MR Imaging”, Radiology 1986; 160:803-810.
Respiratory triggering attempts to synchronize the MR measurement with the respiratory cycle of the freely breathing patient and to limit the measurement to the relatively quiet (i.e. low-movement) phase of the respiratory cycle at the end expiration. For this purpose, the breathing of the patient is detected as a physiological signal with a breathing sensor (for example a pneumatic breathing sensor). After an initial learning phase of the trigger algorithm in which the conditions which should initiate a “trigger” are determined, a “trigger” is thereby generated as soon as the predetermined trigger event (for example a defined phase of the respiratory cycle) is detected.
As a result of the “trigger”, the MR sequence acquires a portion of the data (which portion is predetermined in turn) of one or more slices of the examination subject. The “trigger” thus initiates the acquisition of predetermined data in the examination region. After such a predetermined data packet has been acquired, the data acquisition stops automatically until the trigger algorithm generates the next “trigger”. A second data packet is thereupon acquired. This workflow is continued until all data of all slices of the examination region to be examined are acquired. A slice means the partial region of the examination region that is excited via a particular selective excitation pulse of the sequence. In two-dimensional (2D) sequence techniques (that are particularly important in connection with the present invention), the examination region is most often divided into one or more groups of slices parallel to one another.
The trigger event is normally selected such that “triggers” are generated during expiration, and such that the data acquisition is limited to the relatively low-movement phase of the respiratory cycle at the end expiration. One trigger event is normally generated per respiratory cycle. A respiration-triggered sequence accordingly acquires data of a particular slice of the examination region once per respiratory cycle.
In respiratory triggered MR measurements, the repetition time (TR)—thus the time between two excitations of the same slice—is not fixed, but rather varies with the respiratory cycle of the patient. The repetition time TR is an important contrast-defining parameter in magnetic resonance imaging. A respiratory triggered magnetic resonance imaging, is characterized by an effective repetition time TReff that is equal to the mean respiratory cycle of the patient. The respiratory cycle of the patient is subject to severe individual (and also disease-dependent) fluctuations and typically amounts to between 3 and 6 seconds. Respiratory triggering is therefore preferably used for those sequences in which the desired repetition time TR lies within this range. A few typical examples are T2-weighted imaging with TSE (TSE: Turbo Spin Echo) sequences and diffusion-weighted imaging with spin echo EPI sequences (EPI: “echoplanar imaging”).
In the following, an acquisition module designates a partial sequence that is executed to excite an individual slice of the examination region and the subsequent data acquisition of the excited volume. For a complete data acquisition of a slice, multiple acquisition modules of this slice are normally necessary. In respiratory triggering, these modules are executed in different respiratory cycles. In order to achieve an acceptable efficiency in the measurements in spite of the relatively long effective repetition times, successive data of multiple slices (instead of only one) are acquired per respiratory cycle, for example, and thus acquisition modules of different slices are executed during one respiratory cycle. The execution of acquisition modules after a “trigger” is designated as an acquisition phase in the following. The duration of the acquisition phase after an individual “trigger” is designated as an acquisition duration Tac_p. The complete acquisition of the data of a slice normally takes place in multiple acquisition phases.
In the acquisition of image data, it often occurs that nuclear spins of a specific tissue component (fat tissue, for example) emit a strong signal. In comparison to other tissue types, fat tissue thereby appears very intensely in the generated images, such that a correct diagnosis generation can by hindered by this. Therefore, a number of techniques have been developed in order to suppress the signal of fat tissue (for example by spectral saturation). It similarly occurs that nuclear spins in specific regions (for example directly adjacent to the examination region to be examined) emit a signal that interferes with the desired acquisition. Techniques have also already been developed to suppress such signals, for example by spatial saturation.
To suppress such unwanted signals in a data acquisition by means of an acquisition module, one or more preparation modules are (normally) switched before each acquisition module. Each preparation module normally includes a radio-frequency excitation or inversion pulse as well as spoiler gradients to dephase the transverse, unwanted signal components.
In the following, a problem that leads to an insufficient suppression of the unwanted signal (here fat) should be explained in the example of fat suppression with spectrally selective saturation pulses. However, the same or, respectively, a similar problem exists in the suppression of other unwanted signal components or in the suppression of fat with other methods.
Fat delivers a very intensive signal (for example in T2-weighted turbo spin echo images) that can outshine other signals and thus can hinder the finding [assessment] of various illnesses. In MR images that are acquired with EPI sequences, fat is shifted in the phase encoding direction relative to the water component. The shifted fat image interferes with the image impression and can superimpose lesions. To suppress fat signals, the fact can be utilized that the resonance frequency of protons that are bound in fat molecules differs by 3.3-3.5 parts per million (ppm), thus by approximately 217 Hz at 1.5 T, from those protons that are bound in water molecules.
To suppress the fat signals, before each acquisition module a spectrally selective excitation pulse can be switched that flips protons that are bound in fat molecules into the transversal plane and does not affect protons that are bound in water molecules. The fat signal that is excited in such a manner is subsequently dephased with a spoiler gradient. In the acquisition module that is switched (activated) immediately afterward, the fat signal accordingly supplies no contribution or supplies only a strongly reduced signal contribution. The duration of an acquisition module in the aforementioned sequence techniques is in the range of the T1 relaxation time of fat (approximately 260 ms at 1.5 T field strength). A significant portion of the fat protons are consequently aligned parallel to the field again after the execution of the acquisition module. These would deliver a signal contribution in a subsequent acquisition module executed immediately after the first acquisition module. This is avoided by the preparation module being switched again before each acquisition module. The spectrally selective radio-frequency pulse of the preparation module is normally not slice-selective. The repetition time of the preparation modules (TR-FAT in the example) is thus shorter than the repetition times of the slice-selective acquisition modules TR_im_ac.
For optimal fat suppression, the excitation flip angle of the preparation module must be selected depending on the repetition time of the preparation modules, as well as the time between the excitation pulse of the subsequent acquisition module (and the field strength-dependent T1 relaxation time of fat). The optimal flip angle is calculated for a dynamic steady state of the fat spins in which the longitudinal magnetization of the fat spins respectively has the same value immediately before the excitation pulse of a preparation module. However, this steady state does not immediately appear after the first preparation module. Rather, the longitudinal magnetization passes through a transcendental state and only approaches the steady state after a series of preparation modules. The fat suppression in the acquisition modules that are executed after the first preparation modules of the entire sequence is consequently not ideal, meaning that fat is not sufficiently suppressed.
In a magnetic resonance measurement that is not respiratory-triggered, this problem exists only once at the beginning of the measurement. In a respiratory-triggered magnetic resonance measurement, the steady state of the fat protons must reestablish after every trigger—thus during each breathing interval—since fat is nearly completely relaxed in the time interval between the last preparation module of the (n−1)-th acquisition phase (after the (n−1)-th trigger event) and the first preparation module of the n-th acquisition phase (after the n-th trigger), since given a typical breathing interval this time interval is approximately five to ten times as long as the T1 time of fat. In particular, all data of a slice (and consequently the MR images that are calculated from these data) whose acquisition modules are executed relatively early after a trigger (slice S1 in FIG. 1, for example) are insufficiently fat-saturated. This can significantly hinder the diagnosis with the aid of these images.
The terms “respiratory gating” and “respiratory triggering” are not used consistently in the prior art. Within the scope of the present invention, respiratory triggering is used to mean a technique that synchronizes the imaging MR measurement with the breathing of the freely breathing patient and attempts to acquire a predefined packet of measurement data during an comparatively quite phase of the respiratory cycle. If a defined slice is excited only once per trigger, as described above the effective TR of the sequence is thus equal to or a multiple of the mean breathing cycle of the patient.
As used herein, respiratory gating means an MR measurement during which the breathing of the patient is detected and associated with the acquired measurement data, but whose repetition rate (in particular its TR, thus the time between the successive excitation of a slice) is independent of the breathing of the patient. Rather, in the case of respiratory gating the repetition rate is controlled by a (sequence) parameter or by an additional physiological signal (not the breathing!), for example an EKG. For respiratory gating, the breathing information is used to repeatedly acquire individual data packets that were acquired during more significant breathing motion, for example, or to predictively acquire especially motion-sensitive k-space lines or k-space lines determining the image impression in an comparatively quite breathing phase or after a diaphragm position was measured which corresponds to such a phase (for example in ROPE—respiratory ordered phase encoding). The problem illustrated above of the insufficient suppression of unwanted signals accordingly exists predominantly in techniques with respiratory triggering, but not in techniques which use respiratory gating, since there measurement can take place continuously (or quasi-continuously in the case of an EKG-controlled measurement).
A sequence for magnetic resonance imaging with which image data of a subject to be examined are acquired and with which signals of nuclear spins of a defined type are suppressed is known from DE 10 2007 011 807, which includes the following steps:
(a) apply a suppression module to suppress signals of the nuclear spins of the specific type,
(b) apply an acquisition module after a wait period (TI) to acquire measurement data,
(c) repeat steps (a) and (b) one or more times, respectively after a repetition time (TR), and
(d) before steps (a), (b) and (c), apply a spin preparation module that shifts a magnetization of the nuclear spins of the defined type into a steady state that is maintained via the application of the subsequent steps (a), (b) and (c).
Alternatively, instead of the spin preparation module the first suppression module can also be designed there so that it comprises an RF pulse whose flip angle is selected so that the magnetization of the nuclear spins of the defined type is shifted into a steady state.
The nuclear spins of the specific type should thus already be shifted into the steady state before the “nuclear magnetic resonance sequence” (Steps (a), (b) and (c)). However, in practice this does not work sufficiently well.